Trump's attorney general pick once sent an unsolicited memo to the Justice Department calling Mueller's obstruction probe 'legally insupportable'

President Donald Trump's pick for attorney general, William Barr, criticized the special counsel Robert Mueller's investigation into potential obstruction of justice as "legally insupportable" in an unsolicited memo to the Justice Department earlier this year.

Barr reportedly told Trump the memo could come up during his confirmation hearing before the Senate.

If confirmed, Barr would have the power to fire Mueller.

Trump frequently characterizes the investigation as a politically motivated "witch hunt" and a fishing expedition, and he has suggested that the "Trump Justice Department" should do more to shield him from Mueller.

William Barr, President Donald Trump's nominee for attorney general, sent an unsolicited memo to the Justice Department in June calling the special counsel Robert Mueller's investigation into obstruction of justice "legally unsupportable" and "potentially disastrous," The Wall Street Journal reported on Wednesday.

In his 20-page memo, which the Journal reviewed, Barr argued that Mueller's obstruction probe is based on an overly expansive reading of the special counsel's powers.

He also wrote that Mueller shouldn't be allowed to demand an interview with Trump about obstruction of justice.

"As I understand it, his theory is premised on a novel and legally insupportable reading of the law," Barr wrote, according to the Journal. "Mueller should not be permitted to demand that the President submit to interrogation about alleged obstruction."

The investigation, Barr said, shouldn't be sanctioned by the Justice Department.

Trump and his allies frequently criticize the Russia investigation as a politically motivated "witch hunt" aimed at undermining Trump's presidency. In particular, Trump's lawyers are said to be worried about the president's legal exposure in the obstruction thread of the inquiry.

The Journal reported that in addition to sending the memo to the Justice Department, Barr also sent it to Trump's attorneys.

Legal experts pointed out that the 20-page document would have taken several hours to write, and that it signals one of two possibilities: either Barr feels very strongly about the obstruction probe, or he was angling for a job.

William Barr was President George H.W. Bush's attorney general. Marcy Nighswander/AP

Matthew Miller, a former Justice Department spokesperson, wrote on Twitter that Barr's memo "raises major questions about whether he should be allowed to oversee the Mueller probe."

While it's true that former officials sometimes relay their thoughts on legal issues to the Justice Department, "20-page memos that are sent to counsel for the subject of an investigation" are "not common, and it doesn't happen by accident," Miller added.

Ultimately, experts said, Barr's views on executive power and his decision to defend Trump in a memo to both the Justice Department and to Trump's lawyers may indicate that if confirmed, he would need to recuse himself from overseeing Mueller.

Mueller's office is investigating Russia's interference in the 2016 presidential election, whether the Trump campaign cooperated with Russia, and whether Trump obstructed justice by firing former FBI director James Comey in a bid to stop those investigations.

Trump nominated Barr a month after the resignation of Jeff Sessions, his former attorney general. Barr had a stint as attorney general from 1991 to 1993, under the administration of President George H.W. Bush, and was working as a lawyer in a private practice at the time he sent the memo.